A conventionally known DC-DC converter is a device for outputting a power by converting DC power to AC power by a switching circuit, transforming this using a transformer, and converting this to DC power by a rectifier circuit. In handling large power, it is general that a full bridge circuit is used. In this full bridge circuit, a switching element at the upper arm side and a switching element at the lower arm side of two pairs of switching elements connected in series, are driven alternately. That is, the switching element at the upper arm side and the switching element at the lower arm side implement reverse on/off drive mutually. However, it became hard switching in turn-on and turn-off of the switching element, which generated a large switching loss, resulting in inferior efficiency.
To solve this problem, a DC-DC converter to reduce the switching loss and to improve efficiency has been disclosed in PATENT DOCUMENT 1. This DC-DC converter is implemented by shifting a phase of on/off drive of one switching element connected in series and on/off drive of the other switching element connected in series, composing the full bridge circuit. In this way, zero voltage switching becomes possible, and reduction of the switching loss is attained. This control system is called a phase shift system.
In addition, in PATENT DOCUMENT 2, there has been disclosed the attainment of enhancement of efficiency and reduction of output ripple, by making either of one set of the switches of a full bridge circuit continuation-on, and the other continuation-off, when a load becomes light, in a resonant circuit.